Why Russia wants Ukraine's coastline and control of the Black Sea?
Russia and Ukraine are competitive global exporters of wheat. 95% of Ukraine’s wheat exports are transported via the Black Sea. Stopping Ukraine from using the Black Sea essentially eliminates competition for Russian wheat exports and kills Ukraine’s ability to export other products destroying their economy.
Currently Russia controls Black Sea coastlines in Crimea (2014) and Georgia (2008). Getting Ukraine’s additional hundreds of miles of coastline would permit Russia more control over the Black Sea.
More control would allow:
Russia is the world's largest exporter of wheat.
Ukraine is the 5th largest exporter of wheat and the largest exporter of seed oils.
War/conflict would affect global grain prices and increase food scarcity in parts of Europe, Northern Africa, and parts of the Middle East.